Mobile communication networks often comprise a plurality of RNCs, each being connected to one or more BSs via a Transport Network (TN). The RNCs are interconnected and connected to other types of network nodes via a Core Network (CN). The BSs wirelessly communicate with communication devices, sometimes also called User Equipments (UEs). The UEs may take the form of mobile terminals.
One important feature of mobile communication networks is the possibility to handover, or handoff, mobile terminals. There exist different reasons why a handover can be beneficial. When, for example, a mobile terminal is moving away from an area covered by its serving BS (“source BS”) and entering an area covered by a non-serving BS (“target BS”), an ongoing call may be transferred to the non-serving BS in order to avoid call termination when the mobile terminal leaves the coverage of the serving BS. In another example, when the capacity for connecting new calls of a serving BS is used up, an ongoing call from a mobile terminal located in an area overlapped by a non-serving BS may be transferred from the serving BS to the non-serving BS. In this way capacity in the serving BS will be freed-up for other mobile terminals that can only be connected to that BS.
Handovers can generally be divided into hard and soft handovers. In a hard handover process the connection to the serving BS is released and only then the connection to the non-serving BS is activated. Hard handovers are intended to be instantaneous so as to minimize the disruption to the call.
In a soft handover process the connection to the serving BSs is retained and used for a while in parallel with the connection to the non-serving BS. In this case the connection to the non-serving BS is established before the connection to the serving BS is broken. Soft handovers may involve using connections to two or more non-serving BSs.
One advantage of soft handovers is that the connection to the serving BS is broken only when a reliable connection to at least one non-serving BS has been established. The chances that the call will be terminated abnormally due to a failed handover are thus low. On the uplink, all the BSs that are actively supporting a call in a soft handover process send the data frame stream that they receive back to the RNC, typically along with information about the quality of the received signal. The RNC examines the signal qualities for all data frame streams and dynamically chooses the stream with the highest quality. Should the signal quality for one stream degrade rapidly, the chance is still good that a strong signal will be available at another BS supporting the call in the soft handover process. In this context, the RNC may combine, or merge, data frame streams received from the mobile terminal via different BSs.